To produce oil and gas out of a well, a wellbore is usually cemented. Typically, a casing string is placed inside a wellbore. A fluid is then pumped through the casing string. When the fluid encounters the bottom of the casing string, the fluid flows out of the casing string and upward through the annulus between the casing string and the wall of the wellbore. The fluid is then allowed to harden, thereby sealing the wellbore.